The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force. It is the only fully functional GNSS in the world, can be used freely, and is often used by civilians for navigation purposes. A GPS navigation device is any device that receives GPS signals for determining a present position. The GPS navigation device calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information, and the general system health and rough orbits of all GPS satellites. The GPS navigation device measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the device's location.